The present invention relates to a semiconductor radiation detector for counting gamma-rays. More particularly, the invention relates to such a detector having an improved gamma-ray energy versus gamma-ray count characteristic for the same dosimetric field.
A Gieger-Muller ("G-M") counter has heretofore been used as a gamma-ray detector. Major disadvantages of the G-M counter are its short life, poor linearity between the gamma count and the dose rate, and the need for a high-voltage supply. With a view to eliminating these defects, semiconductor radiation detectors which take advantage of the characteristics of the semiconductor were developed and have been used commercially. The heart of this semiconductor radiation detector is a wafer of a semiconductor, such as germanium (Ge) or silicon (Si), into which lithium (Li) is diffused to increase the specific resistance of the wafer so that a depletion layer will form upon irradiation with gamma-rays. When gamma-rays penetrate through the depletion layer, secondary electrons are produced as a result of any one of three gamma-ray interactions with the semiconductor, such as the photoelectric effect, the Compton effect or electron-pair production. The resulting secondary ions further interact with the lattice atoms in the wafer to create electron-hole pairs which are converted to current pulses. The dose of gamma-rays can be measured by counting these pulses.
The gamma-ray dosimeter is used principally for counting the number of radioactive gamma-rays. However, for reasons discussed below, the conventional semiconductor radiation detector counts different numbers of pulses for individual gamma-rays in the same dosimetric field if they have different levels of energy. Consequently, the pulse count output of the detector is not the correct indication of the dose of gamma-rays in the dosimetric field of interest.
The operating theory of the semiconductor radiation detector is illustrated in FIG. 3: a p-type silicon substrate 1 is provided with an n-type layer by, for example, the diffusion process, to create a p-n junction. This p-n junction is given a reverse bias voltage V.sub.B that is applied between electrodes 3 and 4 to produce a depletion layer 5 in the substrate. When incident gamma-rays 6 penetrate into the depletion layer 5, secondary electrons 7 are generated as a result of any one of the three gamma-ray interactions with the semiconductor, such as the photoelectric effect A, the Compton effect B or the electron-pair production C. The produced secondary electrons further interact with lattice atoms in the semiconductor to form electron-hole pairs 8, which are converted into current pulses 9 and are counted by a counter 10 incorporating an amplifier. Part of the incident gamma-rays emerge from the depletion layer 5 as scattered gamma-rays 11.
The gamma count C, or the number of pulses for a unit dose rate, is given by: ##EQU1## wherein K: a constant
.mu.si: the absorption coefficient of the detector (Si) PA1 .mu.air: the absorption coefficient of air PA1 l: the thickness of the depletion layer penetrated by incident gamma-rays PA1 S: the area of the depletion layer penetrated by incident gamma-rays PA1 E: the energy of the incident gamma-rays.
As is self-evident from formula (1), for given values of l, the thickness of the depletion layer parallel to the plane of the silicon substrate 1, C varies inversely with E, or stated more specifically, a smaller value of C results from gamma-rays of higher energy. There is, therefore, deterioration in the quality of radiation detection because such detector's sensitivity is not independent of the energy of gamma-rays in the same dosimetric field.
The principal purpose of the invention, therefore, is to provide a semiconductor radiation detector that has an improved quality of radiation detection and which produces the same number of current pulses irrespective of the energy of individual gamma-rays within the same dosimetric field.